First, I am not sure you understand, while I may be a pacifist now (Because that is what my Lord commands me to be under the New Testament), I will not be a pacifist when I return in spirit form in following Jesus Christ into battle at His 2nd Coming.
Second, I am not against what happened in the Old Testament at all. I believe God commanded the eradication of men, women, and children who were evil and extremely sinful who were going to hurt the Israelites both physically and spiritually. But God does not operate exclusively thru the nation of Israel like He once did, though. There is no chosen nation that is carrying out God's judgments anymore. There is no more priesthood or a temple for us to sacrifice animals to anymore. Jesus is our perfect sacrifice. The Temple veil has been torn and the Gentiles do not have to become Israelites to be in favor with God.
Anyways, as for your question:
Q: Why did God use men to fight battles?
A: For the same reason God allowed men to be blessed with great wealth in the Old Testament (But not in the New Testament). It was to show that God is rewarder to those who seek after Him (in the fact that they could see His blessings) and that God showed his people that He cared about them in receiving justice when evil was done against them and or was a threat to their good way of life.
Q: Why did God not fight the battles Himself?
A: God actually was the One who made them to win their battles or not. If the Israelites were not in favor with the Lord, they would lose battles and be conquered. If the Israelites were right with God, they would win battles and have major victories. But God did on many other occasions fight the battles without using His people as an arm of justice, too. God used a global flood to destroy the wickedness that filled the entire planet. Hezekiah prayed for protection against an army (whereby they had no hope of surviving on their own power); And a Messenger of the Lord came and destroyed the entire army that was against them. There are many many more examples of this, too. However, one thing you won't find in the New Testament is Jesus and the apostles fighting their enemies, though. Another thing you will not find is a doctrine or teaching on how the believer is supposed to behave in the art of war, either. No commands on self defense. Nothing. On the contrary, Jesus said love your enemies; And Paul said be not overcome by evil, but overcome evil with good. Violence doesn't sound like overcoming evil with good (If you ask me).